/Engine/Source/Runtime/PreLoadScreen/Public/PreLoadSlateThreading.h
The Slate thread is simply run on a worker thread. Slate is run on another thread because the game thread (where Slate is usually run) is blocked loading things. Slate is very modular, which makes it very easy to run on another thread with no adverse effects. It does not enqueue render commands, because the RHI is not thread safe. Thus, it waits to enqueue render commands until the render thread tickables ticks, and then it calls them there.
class FPreLoadScreenSlateThreadTask : public [FRunnable](API\Runtime\Core\HAL\FRunnable)